The answer depends on what characteristic of the data set you wish to represent. I suggest you use the number of observations, 7.
Measuring Cylinder.
A meter ruler or a tape measure
If you mean measuring the height, that would be expressed in meters.
you would use gallons to measure the capacity of a bathtub
Use a variable to represent the situation. For example: John is 10 years older than Frank. Frank would be represented by x John is x+10
You use negative numbers to represent subtractions (10-5 is same as 10 + -5) Also, you use these to represent things that can be negative.
You wouldnt use a unit, just a numerical value such as 5 or 10.
I10
-10
A balance.
Identify the SI until you would use the measured the width of your classroom
It would depend on the person and what the numbers represent